LED chips can generate visible or non-visible light in a specific region of the light spectrum. The light output from the LED may be, for example, light in the blue, red, green, non-visible ultra-violet (UV), and/or near-UV spectral regions, depending on the material composition of the LED. When it is desired to construct an LED light source that produces a color different from the output color of the LED, it is known to convert light output from the LED having a first wavelength or wavelength range (the “primary light” or “excitation light”) to light having a second wavelength or wavelength range (the “secondary light” or “emission light”) using photoluminescence.
Photoluminescence generally involves absorbing higher energy primary light with a wavelength-conversion material such as a phosphor or mixture of phosphors. Absorption of the primary light can excite the wavelength-conversion material to a higher energy state. When the wavelength-conversion material returns to a lower energy state, it emits secondary light, generally of a different wavelength/wavelength range than the primary light. The wavelength/wavelength range of the secondary light can depend on the type of wavelength-conversion material used. As such, secondary light of a desired wavelength/wavelength range may be attained by proper selection of wavelength-conversion material. This process may be understood as “wavelength down conversion,” and an LED combined with a wavelength-conversion structure that includes wavelength-conversion material, such as phosphor, to produce secondary light, may be described as a “phosphor-converted LED” or “wavelength-converted LED.”
In a known configuration, an LED die such as a III-nitride die is positioned in a reflector cup package and a volume, and a conformal layer or thin film of or including wavelength-conversion material is deposited directly on the surface of the die. In another known configuration, the wavelength-conversion material may be provided in a solid, self-supporting flat structure, such as a ceramic plate, single crystal plate or thin film structure. Such a plate may be referred to herein as a “wavelength-conversion plate.” The plate may be attached directly to the LED, e.g. by wafer bonding, sintering, gluing, etc. This configuration may be understood as “chip level conversion” or “CLC.” Alternatively, the plate may be positioned remotely from the LED by an intermediate element. Such a configuration may be understood as “remote conversion.”
Depending on the desired far-field pattern of the light output from any chip plus converter configuration, one drawback associated with wavelength-conversion plates may be that a certain amount of light can escape through the sides of the converter during the conversion process (side emission). Side emission can result in reduced efficacy and/or inhomogenous light distribution with respect to angle. Also, the heat generated during any conversion process can reduce efficacy of the system, particularly in instances where a wavelength-conversion plate is used in high brightness/power applications.
In some applications, the side emission issue has been addressed by casting a ceramic in silicone layer around the sides of the conversion plate. For example, a TiO2 in silicone casting may be formed by mixing TiO2 powder into silicone, and then disposing the resulting material around an LED chip and a wavelength-conversion plate. The silicone in the cast material may then be cured to create a solid reflecting layer around the emitting surface of the wavelength-conversion plate. As a result, only the top surface of the wavelength-conversion plate may be exposed to emit light. Light emitted to the side of the conversion material is reflected by the reflecting material.
Although this solution can effectively address side emission, it requires ceramic in silicone layers to be individually cast around the wavelength-conversion plate used in each lamp package. This can add to the complexity of the lamp manufacturing process. In addition, the ceramic in silicone material may be overfilled during casting, causing it to cover a portion of the top surface of the wavelength-conversion plate and potentially reduce light output. Conversely, the ceramic in silicone material may be under filled during casting, leaving areas where side emission from the wavelength-conversion plate is still possible.
In addition to the aforementioned optical problems, the use of ceramic in silicone materials may also impose limitations on the thermal management of a system into which it is incorporated. For example, in systems where a significant amount of heat is generated (e.g., high power/brightness applications), thermal breakdown of the silicone (or other organic material) in the reflecting layer may occur. Moreover, because the silicone (or other organic material) in the casting has low thermal conductivity, it may not be able to conduct sufficient heat away from the LED package and/or wavelength-conversion plate, which may result in overheating.